1. Field of the Invention
The present invention generally relates to a communication apparatus, and in particular relates to a telephone line interface between a facsimile and a copier, a printer, and/or a card MODEM or the like that has a facsimile function.
2. Discussion of the Background Art
Recently, communication standards adopted by European countries have been unified into the Common Technical Regulation 21 (hereinafter referred to as the CTR 21).
However, the characteristics of a direct current that flows through a network control section of a communication apparatus, e.g., a facsimile, are different from those of the conventional standard. Namely, a direct current is required to have nonlinear characteristics that arise due to limits on a direct current that flows through a direct current circuit.
Further, since communication standards adopted by many countries (excepting Europe) are not uniform, linear characteristics are required in several countries, and the direct current characteristics must be changed to those of a destination country.
Therefore, two different baseboards for generating direct currents with different characteristics are necessary in the network control section of communication apparatus when communication with another country that has adopted a different communication standard is performed. Namely, a network control unit having both a background direct current circuit which does not limit a direct current to a prescribed value and that having the direct current circuit for the CTR 21 use, i.e., which limits a direct current, should be provided.
Further, the characteristics of a network, except for external line network connections executed by a public exchanger based on the Post Telephone Telegram standard (hereinafter referred to as the PTT standard) of the country, are sometimes not based on the PTT standard of the applicable country.
Specifically, an internal line exchanger that operates with a constant current is included among internal line exchangers, such as a small business use internal exchangers (hereinafter referred to as a PABX), a home PABX, and large scale exchangers employed in a business as a private line.
When using such an internal line exchanger, it has been shown that it is impossible to form a network connection between a communication apparatus and an internal line exchanger if the communication apparatus operates at a current value in excess of the prescribed limit upon the direct current. This is because the network characteristic of the internal line exchanger generally depends upon the manufacturer.
Further, there are strict standards for parameters related to control operations of the general public line networks above and beyond other networks. However, parameters related to control operations of other networks can be, to a large degree, set in consideration of general-purpose uses. Thus, the above-mentioned current limitation is generally not applied in internal line exchangers.
For example, the Japanese Patent Application laid Open No. 8-335994 describes technology related to the above-mentioned apparatus. The technology changes the characteristics of the direct current based on a country by connecting resistors in parallel to a photo relay. Thereby, it is possible to generate a direct current in accordance with a standard of the telephone line of an applicable destination country using a direct current generating circuit with a simple construction.
However, the above-mentioned related technology can not generate a direct current that includes nonlinear characteristics that limit a current flowing through the circuit to a prescribed value. Further, it is not necessary (or, in some cases, desirable) to directly limit a current that flows through a circuit when controlling a network except when the network is the general public line network. This is thus a source of problems.